1. Field of the Invention
The present invention relates to the construction of and signal transmission means for an electromagnetic flow meter for industry.
2. Description of the Related Art
A conventional electromagnetic flow meter is fundamentally based on the remote type construction as shown in FIG. 14. That is, a sensor has a coil for generating a magnetic field, a core and a pair of electrodes provided in a measurement conduit therein, and a transmitter provided separately outside the sensor supplies an exciting current to the coil and transmits an instantaneous flow rate, an integrated pulse signal and status signals such as a range signal, a flow direction signal, an anomaly indicating signal and so on in the analog and digital signal form by amplifying and transforming an potential detected by the electrodes of the sensor. An integral type electromagnetic flow meter, an electromagnetic flow meter having the constitution wherein a sensor part and a transmitter are assembled together, is known. But the signal transmission method of such an integral type electromagnetic flow meter is the same method as that of the above-mentioned remote type one. The conventional electromagnetic flow meter has the followings disadvantages.
As for the conventional remote type electromagnetic flow meter:
(a) This type flow meter has a high installation cost because it necessary to provide such elements as highly specified expensive cables and a metal conduit pipe to contain the cables to prevent the signal line from being apt to suffer noises induced by the exciting line or other power apparatus due to tens, uV of the low signal level and the high impedance at the minimum span. PA1 (b) Since a flow meter is usually installed in a high humidity environment, degradation of electric insulation occurs at terminal portions of signal cables with high input impedance, which often brings about measurement errors or measurement impossibility because of the resultant noise generation. PA1 (c) The length of a cable has restriction in order to keep good S/N ratio and the allowable length is about 300 m. A longer cable than that length makes the measurement impossible. PA1 (d) Cost increase or reliability decrease is occasionally brought about by the complicated constitution of a flow meter due where it is necessary to use such a technique as the shield drive method to solve effects caused by span decrease due to an capacitance of a cable in case of measuring low conductivity fluid. PA1 (a) In the case where such signals as a pulse signal, a range signal, an alarm signal, an anomaly indicating signal and so on are separately to be outputted from the flow meter in addition to the flow rate signal, composing the flow meter of the integral structure is very difficult since the part containing electronic circuits becomes large due to an increase in size of a terminal portion and a line inlet or outlet portion for connections to outer circuits. A conventional integral structure of the flow meter is usually realized by composing the electronic circuits at the sacrifice of a part or the whole of the above-mentioned transmitting functions. PA1 (b) In case many output signals and a long distance transmission between the flow meter and a signal receiving instrument are required, a high cost for cabling and construction become problems. PA1 (c) In order to compose the transmitter having such multiple and high quality functions for transmitting signals as mentioned in (a), the number of necessary parts becomes large, which decreases the reliability of the whole flow rate measurement system. PA1 (a) In a transducer, those minimum elements necessary for the transducer to function as an electromagnetic flow meter are built-in, and in a signal processor provided separately from the transducer, subsidiary functions for supporting the functions of the above-mentioned autonomous transducer are provided. A digital transmitting means is applied to transmit multiple kinds of information between the transducer and the signal processor. PA1 (b) The differences of reliability levels in the transmission means are established so that the most reliable analog current signal transmitting means is provided for the flow rate signal as the main signal and less reliable transmitting means are provided for other subsidiary signals. PA1 (c) To decrease a transmitting cable cost, multiple kinds of information are sent by a cable in which digital signals of other information are superimposed to the analog signal. PA1 (d) To increase the reliability of the transmission of the analog signal, the analog current signal such as the flow rate signal is passed through the signal processor in the receiving side without receiving any signal processing. The superimposed digital signals are taken into the signal processor by an alternating current coupling means in the signal processor and processed for operation and control. Thereby, the normality of the flow rate signal as the main signal is kept even if any trouble occurs in the signal processor. This maintains the high reliability of the flow rate signal. PA1 (e) A transmitting part is also incorporated to the transducer by the alternating current coupling, and then the normality of the analog signal transmission is kept even if any trouble occurs in the digital signal transmitting part, which again maintains the high reliability of the flow rate signal. PA1 (f) A power source of a transducer is fed by a direct current power source through the signal processor. Thus, the wiring and installation cost can be lowered since the power line, unlike the case of using the commercial electric power, can be bundled in a multi-conductor cable because there is no potential of cross talk which induces noises from the power line to the signal lines. PA1 (g) A plurality of transducers can be operated by one signal processor since each of transducers can autonomously function as an electromagnetic flow meter. Since the analog current signal of each transducer is passed through the signal processor, it does not receive any effect by the signal processor. And the subsidiary information can be taken into the signal processor by the time sharing method since the subsidiary information is sent by the digital transmitting means through the alternating current coupling. PA1 (h) The calculation processing can be easily executed with respect to the digital signals even if one signal processor gathers the signals from plural transducers. For example, the summations or the differences among the flow rate signals from plural transducers are easily obtained. PA1 (i) Data relating to the flow rate of the fluid to be measured and data for controlling the transducer and the signal processor are transmitted as digital signals between the transducer and the signal processor, and conformity between the data stored in said memory of the transducer and the data stored in a memory of the signal processor is examined to diagnose the data transmission between the transducer and the signal processor. PA1 (j) The base band method or the frequency shift keying (FSK) method which give the best actual results are applied as the digital transmitting method of the subsidiary information.
As for the conventional integral type electromagnetic flow meter: